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The trace element iodine is an essential nutrient for humans. According to some nutrition professionals, good sources of iodine are iodized salt, seafood and dairy products, while plants might be a less reliable source as their iodine content depends on the varying iodine distribition of the underlying soil.

Should vegans incorporate special, iodine-rich foods, or is it sufficient to rely on iodized salt and plants in common doses?

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The UK Vegan Society calls iodine "a slightly challenging mineral".

While seaweed is a very rich source, there are worries that it may contain too much, and also be contaminated with harmful heavy metals and other pollutants. Both too much and too little iodine are problematic, and it's very hard to know exactly how much you are absorbing from your diet, since soil content information is rarely available to consumers and other food chemicals may interact with iodine in ways that are not fully understood.

Here's their advice:

Supplements are the most reliable way to get just the right amount of iodine in your diet. For example, our VEG1, contains 150 mcg per tablet, which is the adult RDA. If you are worried about overdosing on iodine, try to analyse how much iodine is in the vegetables you eat. This may be possible if you buy your vegetables locally, or some manufacturers may know.

Iodized salt is another option, but be careful. Check the label to see exactly how much iodine, and sodium, your table salt contains. For example, your salt may not be iodized at all. Alternatively, you may find just half a teaspoon of table salt contains your RDA. As little as four teaspoons may contain your daily upper safe intake of iodine.

There is no easy way to know how much iodine is in your food. Many crops are low in iodine, because they are grown in areas with iodine-deficient soil. Sea vegetables can have too much iodine, and be contaminated with toxins. The BDA says: "do not use seaweed or kelp supplements as an iodine source" because of these uncertainties.

Dietitians recommend that you take a daily iodine supplement, to be confident you are getting just the iodine you need. Vegans with thyroid issues should discuss iodine intake with their doctor before supplementing.

Further information in their leaflet

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According to Wikipedia (and common knowledge):

Where natural levels of iodine in the soil are low and the iodine is not taken up by vegetables, iodine added to salt provides the small but essential amount of iodine needed by humans.

However, this article from Harvard tells us to pay attention when consuming iodized salt along with "hidden salt".

Iodized salt can theoretically provide all the required daily recommended iodine quantity, but this will lead to consumption of a rather great quantity of salt:

To get all your iodine from salt, you would need more than half a teaspoon of iodized salt a day. That's two-thirds of the daily allotment of sodium (1,500 milligrams) recommended by the American Heart Association.

In the mean time:

between 75% and 90% of sodium in the average American's diet comes from prepared or processed food, and most food companies don't use iodized salt

So, it is recommended to obtain iodine mostly from food, if possible:

[...] you can cut back on salt and not worry about losing out on this important element. Ocean-caught or ocean-farmed fish and shellfish tend to be naturally rich in iodine. Other good sources include milk, cheese, yogurt, eggs, and vegetables grown in iodine-rich soil. Multivitamin pills that also contain minerals usually provide 150 micrograms of iodine.

Conclusion: iodized salt and plants in common doses should be fine, but multivitamin pills can be considered when in doubt about reaching the required iodine recommended quantity.

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